The present invention concerns a windmill with a rotor of the usual type which is normally configured with three blades extending from a blade hub on the main shaft of the windmill housing. The mill housing is disposed in a rotatable manner on the top of the mill tower, and with so-called front-runners the system is arranged so that the rotation of the mill housing is controlled to maintain the blade rotor facing towards the wind, i.e. so that the wind first hits the rotor and thereafter the tower.
The windmill blades are predominantly configured as steel constructions where efforts are made to keep these both as light and as rigid as practically possible, the result being a compromise in that the blades are provided with considerable rigidity, but at the same time also a certain elastic resilience which will naturally manifest itself with blades of greater lengths, e.g. 15-30 m. The wind pressure in against the blade rotor, where the blades turn their broad sides against the wind, will thus give rise to an elastic rearwards flexing of the blades. With rotating blades, this situation is alleviated to a considerable degree by two different circumstances, i.e. partly that the centrifugal force which simultaneously arises on the blades seeks to straighten out the blades, and partly that the blades, as a consequence of their movement transversely to the wind, are influenced by a wind direction which is not facing directly in towards the broad surface of the blades. However, there will still arise a wind component which seeks to force the blades rearwards.
The consequence of this related, elastic flexing rearwards by the blades, is that during their movement past the tower, the stronger the wind the closer the outer ends of the blades will come to the tower. Since the windmills shall preferably also be able to function effectively in strong winds, it is thus necessary for the blade rotor, seen in the direction of the mill housing""s main shaft, to be positioned so far forwards that the blades, during their rotation, at no time can be exposed to influence by the wind to such a degree that they make contact with the tower with associated risk of serious accidents or direct damage. Various items of security equipment can be used for the braking of the rotor in strong winds, but in such cases it must be taken into consideration that the more the rotor is braked, the more the effective wind pressure against the blades"" broad sides is increased to a considerable degree.
The problem could naturally be solved by the blade rotor being positioned at great distance from the mill tower, but for many reasons this is a very expensive solution. Consequently, attention is focused on the stiffness of the blades, precisely so that these can withstand a high wind pressure without the blade hub having to be disposed at some exaggerated distance from the vertical axis of the tower.
The blades are traditionally constructed as aerodynamic shell profile elements with a long, straight main beam extending out along the blade at its thickest area. This main beam is in itself particularly rigid precisely in the direction which is relevant here, which will be in the direction of the beam""s height, but on the basis discussed there can be reason to undertake closer considerations with regard to the relevant flexibility of the beam, i.e. with regard to extra stiffening.
Two different methods have hitherto been suggested for the solving of the problem, though without either of these having made the provision that the main beam should be stiffened, i.e. firstly that the blade rotor is placed on a main shaft which slopes slightly upwards, and secondly that the blades are mounted in a forwardly-inclined manner, which solutions can be briefly commented on as follows:
When use is made of an upwardly-sloping main shaft, the whole of the rotor plane is tipped upwards/rearwards, so that in their rotational movement in the space above the tower, the blades can freely swing in over the contour of the underlying tower, while the passage of the blades below will take place at increased distance from the tower. The mill can thus operate with a considerable free distance between the tips of the blades and the side of the tower, even though the blade hub is positioned more or less close to the tower, in that the blades below will thus swing free of the tower even with strong rearwards deflection, while above they will obviously swing free of the tower, even though they are deflected rearwards completely in over it.
With this arrangement, the rotor plane will be inclined in relation to the vertical, and it has been ascertained that merely this condition in itself appears generally unattractive aesthetically, because for an immediate consideration it is rather illogical. It is known that there exists a deep emotional engagement surrounding the sensible utilisation of the wind power, and the condition named here is enough in itself to have a misleading effect on the debate in this field.
The same applies with said second solution outlined above, i.e. where although the blades rotate in the vertical plane, they assume a xe2x80x9cconexe2x80x9d formation in the forwards direction, i.e. they are placed in such inclined retaining parts on the blade hub that they will not only be simply xe2x80x9cradiatingxe2x80x9d, but each also slightly forwardly inclined, so that in their rotation they described a conical surface. It can also be achieved hereby that the blade hub can be situated at a more or less short distance from the shaft of the mill tower, while the blade tips in a blade rotor rotating in the vertical plane have, however, ample possibilities of being pressed back without hereby colliding with the mill tower. However, this solution is relatively costly, the reason being that it is expensive to arrange the slightly inclined retaining parts on the blade hub, and to which can be added that also the resulting appearance of a conical blade rotor gives rise to objections from the point of view of aesthetical criteria.
While there is a strong case that the invention should be concerned with a further stiffening of the said main beam in order to obtain a picture of a blade rotor rotating in a vertical plane, with the invention there is introduced the modification that the use of said xe2x80x9cstraightxe2x80x9d beams is omitted in the construction of the blades, and use being made instead of xe2x80x9chigh-edge curvingxe2x80x9d main beams, without necessarily speculating further concerning the stiffness of these beams.
The blades in themselves will thus be able to appear as a right-angled extension out from the blade hub, which can more or less gradually go over into a curved extension, e.g. merely along the outer third of the blade, so that the blade tips stand at a greater distance from the tower. A rotor with blades shaped in this way has proved not only to be more effective for the considered objective, but also aesthetically acceptable, in that visually the curving outwards manifests itself very little. For example, it will be quite an attractive result if the blade hub can be placed a half a metre closer to the tower shaft, and an associated outwards curvature of a blade with a length in the order of 25 m will be only slightly visible, namely when the inner, thicker length parts can be seen to work in a plane which appears natural to the observer.
Compared to blades with the said negative coning, the blades according to the invention will distinguish themselves partly in that the rotor hub can be built in a conventional manner, and partly that the hub will be not influenced by substantial bending forces originating from the effect of centrifugal force on the blades. With coned blades, a great part of the straightening-out of the blades will be due precisely to the centrifugal force, so that they must be extra coned in order also to accommodate the rearwards deflection which occurs under wind pressure.
Compared with the said tilted rotors, the invention offers the advantage that the blades are influenced in a uniform manner all the way around, in quite the same way as with conventional rotors, while the blades in a tilted rotor are influenced unevenly, i.e. all depending on whether they are moving upwards or downwards, and are consequently herewith moving respectively with and against the wind, which corresponds to a yawing-error which can give rise to various problems.
As mentioned above, it is characteristic of a blade according to the invention that the main beam itself is partly formed with a curvature in the high-edge plane, but it must be emphasized, however, that the invention is not limited to this, in that by formation in a suitably shaped mould the blade can be given the desired curvature also with wholly or partly beam-free box constructions.